This invention relates to an improvement of a low-pressure mercury vapor discharge lamp, and particularly to an improvement in the stain prevention of the wall of the lamp and in the lumen maintenance of the lamp.
A fluorescent lamp with a metal oxide layer formed between its glass wall and phosphor coating layer to improve the lumen maintenance, was disclosed, for example, in U.S. Pat. No. 3,541,377.
The input wattage to the fluorescent lamp, divided by .pi.Dl where l (cm) is the arc length of the lamp, and D (cm) is the average inner diameter of the lamp, is called the wall load. In the tubular, circular, U-shaped and double U-shaped fluorescent lamps of 0.08 W cm.sup.-2 or above in its wall load which are now practically used, the lumen maintenance was surely improved by providing a metal oxide or phosphate layer between the glass wall and phosphor coating layer. However, according to the experiment by the inventors of this invention, when the fluorescent lamp with a metal oxide layer or phosphate layer provided between the glass wall and phosphor layer was operated at a higher wall load, part of the glass wall was colored yellow, or the so-called stain appeared, deteriorating its appearance.